The present invention relates to a plastic-coated metal plate which is used for forming a car body and has an excellent chipping resistance.
In car bodies of passenger cars and light cars, a part to which equipments such as an engine and a chassis are not installed and which is composed mainly of sheet metal is called xe2x80x9ca shell bodyxe2x80x9d and is composed usually of xe2x80x9ca main bodyxe2x80x9d composed of an underbody, a side member, a roof, a cowl, an upper back and a lower back and xe2x80x9can outer covered partxe2x80x9d such as a hood, a front balance, a front fender, a cowl louver, doors and a luggage (back doors).
These main body and outer covered part have so far been formed by cutting, molding and assembling metal plates into sizes and shapes of the respective constitutional parts, immersing them in a cation electrodeposition paint bath to coat the front sides, back sides and end faces thereof to form primer coat paint films and then applying an intermediate coat paint and a top coat paint on the outside parts.
In recent years, however, step saving, energy saving and a reduction in CO2 in a coating line are strongly desired in coating car bodies. In addition thereto, a chipping resistance and a corrosion resistance of a combined paint film are requested to be further improved. In order to elevate this chipping resistance, it is proposed to provide a barrier coat for forming a visco-elastic paint film in a layer between these paint films, but there are the defects that a coating step is added and the production cost goes up.
Intensive investigations repeated by the present inventors in order to solve the problems described above in car bodies have resulted in finding that the problems described above can be solved at a stroke by using a metal plate coated with a plastic film having specific physical property values, and thus they have come to complete the present invention.
Then, provided according to the present invention is a plastic-coated metal plate for an automobile exterior plate which is a laminate metal plate coated on both faces or one face of the metal plate with a plastic layer, wherein the plastic layer has a surface tension of at least 30 mN/m, an elongation percentage of at least 30% at a tensile rate of 20 mm/minute/20xc2x0 C. and a rupture strength of at least 600 kg/cm2 at a tensile rate of 20 mm/minute/20xc2x0 C.
The plastic-coated metal plate of the present invention for an automobile exterior plate shall be explained below in detail.
The plastic-coated metal plate according to the present invention can be applied to passenger cars and light cars and can similarly be applied as well to trucks, buses, auto-bicycles, special installation automobiles and the like.
Those which have so far been used for car bodies can similarly be used as the metal plate for coating with plastics and, materials therefor include, for example, metal plates of iron, steel, stainless steel, aluminum, copper and alloys containing these metals, and metal plates obtained by plating the surfaces thereof with zinc, zinc/nickel and iron, and they can be used by working into the form of a coil or a cut plate. The metal plates have suitably a thickness falling in a range of usually 0.3 to 2.0 mm, particularly 0.5 to 1.0 mm. The surfaces of these metal plates are preferably subjected in advance to suitable polishing treatment, degreasing treatment and phosphate treatment in order to elevate an adhesive property with a plastic layer and an anticorrosive property.
Plastics used for coating the metal plates shall not specifically be restricted in terms of materials therefor, and there can be used, for example, conventionally known thermoplastic resins or thermosetting resins including polyolefin resins such as polyethylene and polypropylene, polyester resins such as polyethylene terephthalate (PET), polycarbonate resins, epoxy resins, vinyl acetate resins, vinyl chloride resins, fluorine-containing resins, polyvinyl acetal resins, polyvinyl alcohol resins, polyamide resins, polystyrene resins, acryl resins, polyurethane resins, phenol resins and polyether resins. These resins may suitably contain color pigments and extender pigments.
These plastics in a state that they are coated on the surface of the metal plate in a layer have a surface tension of at least 30 mN/m, particularly 35 to 55 mN/m and more particularly 38 to 50 mN/m, an elongation percentage of at least 30%, particularly 50 to 300% and more particularly 60 to 250% at a tensile rate of 20 mm/minute/20xc2x0 C. and a rupture strength of at least 600 kg/cm2, particularly at least 1000 kg/cm2 and preferably 1000 to 2500 kg/cm2 at a tensile rate of 20 mm/minute/20xc2x0 C. (xe2x80x9cmNxe2x80x9d described above is an abbreviation of milli-Newton). If the plastic layer has a surface tension of smaller than 30 mN/m, the adhesive property with an intermediate coat paint film and a top coat paint film which are formed on the surface thereof is reduced, and interlayer peeling is caused in a certain case. Also, if the plastic layer has an elongation percentage of smaller than 30%, the paint film does not become satisfactorily flexible. Further, if it has a rupture strength of smaller than 600 kg/cm2, deformation and cracking rupture of the paint film are liable to be caused by an impact force, and therefore there is the possibility that the chipping resistance is reduced.
A method for allowing the plastic layer formed on the metal plate to fall in characteristic values having the ranges described above includes, for example, methods in which 1) a composition of the plastic layer is suitably selected; 2) the plastic layer is treated with chemicals or organic solvents or coated with a primer, a coupling agent or a surfactant; 3) the plastic layer is subjected on the surface thereof to grafting or colloid treatment; and 4) the plastic layer is subjected to irradiation with UV rays, vacuum discharge treatment, flame treatment, ozone treatment, plasma contact treatment or corona discharge treatment. The physical property values of the plastic layer can readily be controlled in the ranges described above by carrying out these methods suitably in combination. Among them, the corona discharge treatment can be carried out, for example, by carrying out three pass treatment of 40 to 50 KHZ at a maximum output of 1.0 Kw by means of a high frequency power source CG-102 type (manufactured by Kasuga Co., Ltd.).
The surface tension (xcex3S) of the plastic layer is a value obtained by dropping deionized water and paraffin on the surface of the prepared plastic layer to determine a contact angle (xcex8) to the plastic layer and applying it to the following equation. The contact angle (xcex8) can be measured at 20xc2x0 C. by means of CONTACT ANGLE METER (trade name, manufactured by KYOWA KAIMEN KAGAKU Co., Ltd.):
surface tension of deionized water:
xcex3LW=72.8 mN/m
xcex3LWd=29.1 mN/m
xcex3LWp=43.7 mN/m
xcex8w: contact angle of deionized water
surface tension of paraffin:
xcex3LP=24.4 mN/m
xcex8p: contact angle of paraffin
xcex3Sd=xc2xcxc2x7xcex3LP(1+cos xcex8p)2 
xcex3Sp=[(1+cos xcex8w)xc3x97xcex3LWxe2x88x922{square root over ( )}xcex3LWdxc2x7xcex3Sd]2/4xc3x97xcex3LWp 
xcex3S=xcex3Sd+xcex3Sp 
The elongation percentage and the rupture strength of the plastic layer are values obtained by forming a plastic layer on a tin plate by a suitable method, separating it by a mercury amalgam method and then measuring the plastic layer of length 20 mm and width 5 mm at a tensile rate of 20 mm/minute and a temperature of 20xc2x0 C. by means of a tensile tester (TENSILON UTM-II-20, trade name, manufactured by TOYO BOLDWIN Co., Ltd.).
The metal plate can be coated with the plastic film by conventionally known methods. It can be coated, for example, by such a method that a film-shaped or sheet-shaped plastic molded by extrusion molding, injection molding, calendar molding or compression molding is adhered on a metal plate; plastic which is molten by heating is extruded into a film or sheet, and it is adhered on a metal plate by pressing; or powdery plastic is adhered on a metal plate by fluid dipping or electrostatic coating and molten by heating.
The plastic film is coated on the surface of the metal plate positioned at least on the outside of an automobile part but can be coated, if desired, on both surfaces. The plastic layer coated on the metal plate has a thickness falling suitably in a range of usually 1 to 100 xcexcm, particularly 3 to 75 xcexcm and more particularly 5 to 50 xcexcm.
In coating the metal plate with the plastic, an adhesive is preferably applied in advance on either of the metal plate and/or the plastic film or both of them in order to enhance an adhesive property between both. Such adhesive includes, for example, thermosetting adhesives containing at least one resin selected from bisphenol A type epoxy resins, resol type epoxy resins, acryl resins, aminoplast resins, polyester resins, urethane resins and polysiloxane. Further, there can also be used as the adhesive, triazinethiol base compounds such as 2,4,6-trimercapto-s-triazine, 2-dibutylamino-4,6-dimercapto-s-triazine, 2,4,6-trimercapto-s-triazine monosodium salt and 2,4,6-tri-mercapto-s-triazine trisodium salt.
The plastic-coated metal plate for an automobile exterior plate according to the present invention can be prepared by coating both surfaces or a single surface of the metal plate with the plastic layer in such a manner as described above. The plastic-coated metal plate thus produced is cut, molded and joined, whereby there can be formed, the xe2x80x9cmain bodyxe2x80x9d composed of an underbody, a side member, a roof, a cowl, an upper back and a lower back and/or the xe2x80x9couter covered partxe2x80x9d such as a hood, a front balance, a front fender, a cowl louver, doors and a luggage (back doors).
To be specific, the xe2x80x9cmain bodyxe2x80x9d is composed mainly of an underbody, a side member, a roof, a cowl, an upper back and a lower back. The underbody means floor parts for a passenger compartment (cabin) and a luggage boot and is a general term for a front underbody, a front floor and a rear floor. The side member is to combine with a front body, a roof panel and an underbody to form a side face of a cabin and prevent bending and torsion of the vehicle. The cowl is a panel for connecting longitudinal and lateral pillars. The upper back is a panel for connecting right and left quarter panels (rear fenders) at a rear part of a car body to form an outside face of the car body.
All or a part of these parts can be produced by using the plastic-coated metal plate for an automobile exterior plate according to the present invention. For example, the plastic-coated metal plate of the present invention is cut to an intended shape and size and molded by pressing by a conventionally known method by means of a press working machine to produce the xe2x80x9cmain bodyxe2x80x9d. The main body molded in such a manner as described above is coated at least on the outside thereof with the plastic layer, and the end face part and back side of the cut coated metal plate remain uncoated, but the back side part may be coated with the plastic layer in such a manner as described above.
The xe2x80x9couter covered partxe2x80x9d is composed mainly of a hood, a front balance, a front fender, a cowl louver, doors and a luggage (back doors), and in order to form these parts, the plastic-coated metal plate for an automobile exterior plate according to the present invention is cut and molded by pressing by a conventionally known method, followed by combining them by an adhesive, welding and bolting to thereby form the xe2x80x9couter covered partxe2x80x9d. The outer covered part thus obtained is coated at least on the outside thereof with the plastic layer, and the cut steel plate remains uncoated on the end face part and the back side thereof, so that the metal is exposed, but the back side thereof may be coated with the plastic layer in such a manner as described above.
The xe2x80x9cshell bodyxe2x80x9d can be formed by combining the xe2x80x9cmain bodyxe2x80x9d with the xe2x80x9couter covered partxe2x80x9d which are produced in such a manner as described above by using the plastic-coated metal plate for an automobile exterior plate according to the present invention. Equipments such as an engine and a chassis are usually not installed on this shell body in car bodies of passenger cars and light cars.
Only the outer covered part can be formed from the plastic-coated metal plate for an automobile exterior plate according to the present invention, and this can be mounted on a main body produced by a conventional method, whereby the shell body can be formed.
Metal is sometimes exposed in the end face part and the back face part of the shell body produced in such a manner as described above, and therefore in order to elevate a corrosion resistance in these parts, they are preferably immersed in, for example, an electrodeposition paint bath and apply an electric current to form primer paint films on these parts. Either an anionic type or a cationic type can be used as the electrodeposition paint and usually, a cationic type electrodeposition paint providing a good corrosion resistance is preferably used.
Known ones can be used as the cationic electrodeposition paint and, for example, a paint containing a base resin having a hydroxyl group and a cationic group and a block polyisocyanate compound (a cross-linking agent) is suitably used. In this case, conventionally known ones can be used for the base resin and include, for example, reaction products of polyepoxy resins with cationizing agents; products obtained by protonating polycondensation products of polycarboxylic acids and polyamines with acids (refer to U.S. Pat. No. 2,450,940); products obtained by protonating polyaddition products of polyisocyanate compounds, polyols and mono- or polyamines with acids; products obtained by protonating copolymers of acryl base or vinyl base monomers containing a hydroxyl group and an amino group with acids (refer to Japanese Patent Publication No. 12395/1970 and Japanese Patent Publication No. 12396/1970); and products obtained by protonating adducts of polycarboxylic acids to alkyleneimines with acids (refer to U.S. Pat. No. 3,403,088). Among them, a base resin prepared by reacting a cationizing agent with a polyepoxy resin obtained by reacting a polyphenol compound with epichlorohydrin is particularly preferred because of an excellent corrosion resistance thereof. The cationizing agent includes, for example, amine compounds such as primary amines, secondary amines, tertiary amines and polyamines. Further, basic compounds such as ammonia, hydroxylamine, hydrazine, hydroxylethylhydrazine and N-hydroxylethylimidazoline may be used as the cationizing agent and allowed to be reacted with an epoxy group of a base resin, and a basic group formed thereby may be protonated into a cationic group with an acid.
Usually, an electrode position paint can be prepared by blending a base resin with a block polyisocyanate compound, then neutralizing a cationic group contained in the base resin with an acid compound such as acetic acid, formic acid, lactic acid and phosphoric acid and mixing and dispersing them with water. The pH in coating falls suitably in a range of usually 3 to 9, particularly 5 to 7, and the solid concentration falls suitably in a range of 5 to 30% by weight.
The cationic electrocleposition paint can suitably be compounded, if necessary, with a curing catalyst having a corrosion resistance such as hydroxides, oxides, organic acid salts and inorganic acid salts of metals selected from aluminum, nickel, zinc, strontium, lead, zirconium, molybdenum, tin, antimony, lanthanum, tungsten and bismuth; various color pigments; and precipitation preventives.
Electrodeposition is carried out, and a paint film thus formed is cure by heating. Then, an intermediate coat paint and a top coat paint which are known as paints for automobiles can suitably be applied on the surface of the plastic layer on the outer face side of the shell body.
The plastic-coated metal of the present invention described above can provide the effects described below.
(1) The plastic-coated metal plate of the present invention for an automobile exterior plate comprising a metal plate coated with a plastic layer having a surface tension, an elongation rate and a rupture strength which fall in the specific ranges is excellent in a chipping property as compared with conventional painted metal plates and has a good interlayer adhesive property with an intermediate coat paint film and a top coat paint film which are coated on the surface thereof.
(2) The shell body produced by using the plastic-coated metal plate of the present invention for an automobile exterior plate is coated at least on an outside face thereof with the plastic layer, and therefore an electrodeposition paint which is coated at a subsequent step can be reduced in a use amount thereof. In addition thereto, because of a smaller area of the non-electrodeposited part (metal-exposed part), the throwing power is elevated, and the corrosion resistance is improved particularly in the end face part.
The present invention shall more specifically be explained below with reference to examples and comparative examples. Both parts and percentage are based on weight, and a film thickness of a coated film is that of a cured coated film.